Abân ibn Taghlib says: I was in a mosque in Mecca with Imam Jaffari Sadiq. He took my hand and said: “...He will then command a herald who will cry out: This is  Hazrat Mahdi (as), he will rule like Sulayman (as) and Dawud (as). His commands will need no evidence or witness.” 

(Sheikh Muhammad bin Ibrahim Nomani, al-Ghaybah al-Nomani, p. 374)